6 Negative Emissions

Negative Emissions Technologies (NETs)

Each year, only about half the amount of CO₂ we emit to the atmosphere stays there. This is due to the carbon cycle that exchanges CO₂ between the atmosphere, the terrestrial biosphere (vegetation and soils), and the oceans (see figure 1). Rock weathering also removes CO₂ from the atmosphere, but at a much slower rate. A number of NETs have been proposed to work with the carbon cycle and enhance the use of natural sinks:

The planting of trees to fix atmospheric carbon in biomass and soils, termed “afforestation and reforestation” (AR).
Adopting agricultural practices like no-till farming to increase carbon storage in soils.
Converting biomass to biochar and using the biochar as a soil amendment.
Fertilizing the ocean to increase biological activity to pull carbon from the atmosphere into the ocean (iron fertilization).
Adding alkalinity to the oceans to pull carbon from the atmosphere via chemical reactions.
Enhancing the weathering of minerals, where CO₂ in the atmosphere reacts with silicate minerals to form carbonate rocks.

Another strategy, the primary focus of this chapter, is to use carbon capture technology to remove the CO₂ from the atmosphere. The two strategies proposed are:

Bioenergy with carbon capture and storage (BECCS).
Direct air capture (DAC) of CO₂ from ambient air by engineered systems.

Worldwide emissions of CO₂ are approaching 40 GtCO₂ per year. Therefore, to make a real difference in the fight against climate change, NETs must be able to operate on the scale of gigatonnes CO₂ per year. Afforestation and reforestation, currently deployed on the scale of megatonnes CO₂ per year, is the only NET implemented at large scale today. So NETs have a long way to go to reach the gigatonne-per-year threshold.

In addition to scale, the cost, effectiveness, and environmental impact of each net will determine its success or failure. There is a large variation and much uncertainty regarding the performance of each NET.² For example, cost estimates run from about $10/tCO₂ avoided for AR to $1000/tCO₂ avoided for DAC. Questions abound about the effectiveness and scale of the enhanced biological sink options: AR, modified agricultural practices, and biochar.³ Some of these questions are discussed below in the context of BECCS. Perhaps the most controversial NET is iron fertilization, because of its large environmental impact. In order to work, iron fertilization changes the ecosystem of the area of the ocean to which it applies. Among other things, this includes shifts in species composition and oxygen depletion. As a result, marine biologists have made a strong case that iron fertilization is not a viable NET.⁴

Bioenergy with CCS (BECCS)

The IPCC Fifth Assessment Report (AR5) Summary for Policymakers states, “Combining bioenergy with CCS (BECCS) offers the prospect of energy supply with large-scale net negative emissions which plays an important role in many low-stabilization scenarios, while it entails challenges and risks. … These challenges and risks include those associated with the upstream large-scale provision of the biomass that is used in the CCS facility as well as those associated with the CCS technology itself.”⁵

BECCS can be broken down into three major components. The first step is the growing of biomass; photosynthesis converts the CO₂ in the atmosphere to hydrocarbons in the biomass using sunlight as the energy source. The second step involves converting the biomass to either electricity or fuels and capturing the CO₂ emissions associated with the conversion process. The final step is the transport and storage of the captured CO₂. The net result is to produce usable energy in the form of electricity or fuels while removing CO₂ from the atmosphere and storing it in geologic formations. As will be described below, the amount of negative emissions generated from BECCS depends on many factors, and not all processes that convert biomass to commercial energy can claim negative emissions.

Biomass Production

The biomass feedstock for a BECCS process can come from agriculture or forestry residues, waste streams like municipal solid waste, or dedicated energy crops. Today, many biomass processes use residues and wastes because they are the cheapest feedstocks. However, to implement BECCS at a large scale, it will be necessary to develop a supply chain based on energy crops. Energy crops may be herbaceous biomass like switchgrass, or woody biomass like loblolly pine. In developing large-scale energy crops, the issue of land availability is a major concern. There are fears that competition with food production for land will lead to an increase in food prices. Energy crops grown on abandoned or agriculturally degraded lands can help mitigate this issue. Other concerns related to energy crops include biodiversity protection, prevention of soil degradation, and water usage.

While the carbon in the biomass comes from the atmosphere, the biomass feedstocks do have a carbon footprint associated with them. This footprint arises from the fossil carbon inputs associated with the growth, harvesting, and transport of the biomass. Fertilizers added during the growing period not only have CO₂ emissions associated with their manufacture, but because they contain nitrogen, they also emit N₂O, a potent greenhouse gas. There are CO₂ emissions associated with the machines and vehicles used during the harvesting, collection, and transport of the biomass. These emissions of greenhouse gases are generally in the range of 5 to 15 percent of the atmospheric CO₂ stored in the biomass.⁶ The net negative emissions from a BECCS process are the amount of CO₂ that is contained in the biomass minus the CO₂ emissions released during biomass production and processing. In the case of biofuels, the CO₂ released by the use of the biofuels must also be subtracted.

In addition to the CO₂ emissions described above, there can be greenhouse gas emissions associated with land use change. For example, if one decided to grow energy crops on grasslands previously used for grazing, the change would result in greenhouse gases being released from the existing vegetation and soils. These one-time emissions, termed “direct emissions from land-use change,” can be quite large and need to be accounted for. Indirect emissions from land-use change are much more difficult to quantify. Building on the above example, the use of grazing land to grow energy crops may lead to the cutting down of a forest to replace the lost grazing land. It is very hard, indeed maybe impossible, to know how a land-use change in one place may induce a land-use change elsewhere. It is an issue that makes using biomass for climate mitigation or negative emissions somewhat controversial.

Bioelectricity

A little under 1 percent of US electricity production is from biomass (see chapter 2). Most of this use is from inexpensive sources of biomass, generally residues from activities like timber or pulp and paper production. The biomass is combusted in stand-alone boilers, as well as co-fired in coal-fired boilers. Existing coal-fired power plants can co-fire up to 10 to 15 percent biomass with little or no modifications.⁷

The conversion of biomass to electricity is conceptually identical to the conversion of coal to electricity. The biomass or coal is combusted in a boiler to raise steam that drives a turbine/generator that produces electricity. Pollutants are removed from the flue gas before being sent up a smokestack into the atmosphere. There are some important differences in combusting biomass compared to coal. Biomass has a high moisture content, resulting in lower power plant efficiencies. Where a coal-fired power plant may be 40 to 45 percent efficient, a biomass-fired power plant will be only 30 to 35 percent efficient.⁸ Biomass is also very fibrous. To feed coal into a boiler, it is first “pulverized” into small particles. Since untreated biomass is not amenable to pulverization, other approaches, such as pelletization, are used.

The pelletization process involves drying, grounding, and extruding the biomass. The resulting pellets have a higher mass density and lower moisture content than the starting biomass. This yields several advantages: lowering transportation costs, facilitating feeding the biomass into the boiler, and increasing combustion efficiencies in the boiler. Pelletization increases the cost of the biomass feedstock, and the CO₂ emissions associated with pelletization need to be accounted for when calculating the net negative emissions. However, pelletization is generally worthwhile, especially if the biomass is being transported significant distances. Today, biomass pellets are routinely shipped from North America to Europe.

Adding carbon capture to a biomass-fired power plant is very similar to adding carbon capture to a plant that is coal-fired. The biggest difference is the nature of the impurities in the flue gas and their impact on the capture process. The standard procedure today is to remove the contaminants before the flue gas enters the capture facility. Successfully achieved for coal-fired power plants, there is no experience yet on biomass-fired power plants. No major barriers are expected, but the procedures still need to be developed.

In the literature, there are many references to gasification of biomass in the production of electricity, termed Biomass Integrated Gasification (BIG). As with coal, gasification has been promoted as a way to achieve higher efficiencies and lower emissions of pollutants (see chapter 3). Gasifying biomass is more difficult than gasifying coal, because of the high moisture content and the difficulty in feeding the biomass into the gasifier. Since the experience with coal gasification for electricity has been disappointing, it is unrealistic to think that gasification of biomass for electricity will happen anytime soon. I expect any BECCS projects in the next couple of decades to use combustion technology, not gasification technology.

Biofuels

The Energy Policy Act of 2005 mandated that a certain amount of biofuels be used for transportation in the United States. This and subsequent mandates have been met using corn ethanol, which is blended with gasoline. Brazil also has programs for widespread use of ethanol in transportation fuels, with their ethanol produced from sugar cane. There is only one ethanol production facility that employs CCS—the Decatur Project in Illinois (see chapter 5).

The US ethanol program is quite controversial, with many considering it more of a farm support program than an energy program. Because it is very energy intensive to grow and process the corn, the biomass has a large carbon footprint. Many studies show that adding corn ethanol to gasoline results in very little, if any, carbon reductions. As a result, new policies and research efforts are trying to develop cellulosic biomass as feedstocks for biofuels, which would result in much greater carbon reductions than corn ethanol. Examples of cellulosic biomass are agricultural residues, as well as the type of energy crops discussed earlier in this chapter. Today, there are no economical pathways to turn cellulosic biomass into biofuels on a large scale.

Electricity is a carbon-free energy carrier, so using electricity produced from biomass creates no additional CO₂ emissions. However, most biofuels contain carbon, so they will emit CO₂ when consumed. This makes it very hard for biofuels to be carbon negative. Instead, they can approach being carbon neutral, which is still a big improvement over today’s transportation fuels. The one biofuel that could be carbon negative is hydrogen, which, like electricity, is a carbon-free energy carrier. While the use of hydrogen as a transportation fuel has been studied for decades, there are significant barriers to moving in this direction.⁹ The future of biofuels in at least the near-term is to produce liquid hydrocarbons that fit in well with our current transportation infrastructure. These biofuels can significantly lower or even eliminate the carbon footprint of transportation fuels, but cannot generate significant net negative emissions. Only BECCS-produced electricity or hydrogen can provide significant net negative emissions.

Direct Air Capture (DAC)

Imagine having a bowl of marbles on your desk. It contains 400 red marbles and 3600 blue marbles. Your job is to remove the red marbles. This represents removing CO₂ from the flue gases of power plants or industrial facilities. Now imagine a much bigger bowl containing one million marbles. 400 are red and the rest are blue. Once again, your task is to remove the red marbles. This represents the removal of CO₂ from the atmosphere, termed Direct Air Capture (DAC). The latter task is much more difficult than the former, just as DAC is much more difficult than carbon capture from flue gases.

The question for DAC is not whether we can suck CO₂ out of the air, but whether we can do it economically on a large scale. Commercial technology for removing CO₂ from the air has been in use for over seventy years. In a cryogenic oxygen plant, air is liquefied and distilled to produce high-purity oxygen. At the beginning of the process, CO₂ is removed from the air in order to prevent it from forming dry ice and clogging the heat exchangers. Other commercial air capture applications include removing excess CO₂ from the air in spacecraft and submarines.¹⁰ These air capture applications use absorption or adsorption technologies (see chapter 3) and simply remove the CO₂ from the air. DAC uses the same technologies, but has the additional requirement of recovering the CO₂ at high purities. This added requirement adds complexity and costs.

The basic technological approaches for DAC are similar to capture from flue gases, but the engineering challenges are somewhat different because of the difference in CO₂ concentrations: 3 to 20 percent in flue gases versus 0.04 percent in the air. To illustrate this, we will compare carbon capture from a flue gas with a 12 percent CO₂ concentration (CCS case) with carbon capture from the air (DAC case).

The concentration of CO₂ in the DAC case is three hundred times smaller than the CCS case. As discussed in chapter 3, concentration matters in determining the degree of difficulty of carbon capture. One way to quantify this is by calculating the minimum work. The minimum work in the CCS case is 43.8 kWh/tCO_2, compared with 133 kWh/tCO₂ in the DAC case. These calculations assume a 90 percent capture rate in both cases and consider only the separation work without compression. From this minimum work perspective, DAC is three times as difficult as CCS. However, this is only part of the story.

The Sherwood Plot (see figure 6) provides another perspective for comparison. This empirical relationship states that the more dilute a target material in a feed stream, the higher the cost of removing that material. What drives this relationship is the fact that the larger dilution in the feed corresponds to more material needing to be processed. For DAC, three hundred times more air must be processed compared to CCS. This drives up the relative cost of DAC to CCS beyond the factor of three derived from the minimum work calculation.

Figure 6 The Sherwood Plot

*Source:* Adapted from King et al., *Separation and Purification: Critical Needs and Opportunities* (Washington, DC: National Academy Press, 1987), 9.

I have followed DAC closely ever since I saw a press release in 2003 from Columbia University that stated that “[t]hey estimate that the cost of trapping carbon dioxide from air could eventually be less than 25 cents per gallon of gas, with the potential for better and cheaper methods in the future.”¹¹ This is a preposterous statement. Note that 25 cents per gallon of gas translates into $25/tCO₂. Over the years, the proponents of DAC have made many claims about how cheap DAC can be, which led me to write a paper with several coauthors in order to bring some rationality to the discussion. For the reasons laid out above, we concluded “that air capture will cost on the order of $1,000/t of CO₂.”¹²

I have tried to rationalize why there is such a discrepancy in outlooks for DAC. As noted at the beginning of this chapter, the idea of a CO₂ air purifier is seductive; if cheap enough, it would be the perfect solution to the climate change problem. Just as the concept of “peak oil” once made so much sense (see chapter 2), so too does the concept of a CO₂ air purifier. People want to believe in DAC because it is a simple concept that solves so many problems. The proponents focus on the minimum work calculation, which shows that DAC is only a factor of three more than CCS. They have faith that this gap can be bridged through innovation and ingenuity. At least four companies were formed over the past decade to commercialize DAC. In May 2017, Climeworks started up a 900 tCO₂/year facility in Switzerland.¹³ However, none of this activity has given me any reason to revise my assessment. In fact, the data that I have gathered from these activities supports my analysis. The hard engineering reality of making DAC work at scale, such as the small driving forces and the massive amounts of air to be scrubbed, makes it a very expensive proposition. Concentration does matter. The best way to remove CO₂ from the air is to not release it into the air in the first place.

6
Negative Emissions